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Partial density of states (PDOS) of Ag12O6 phase is calculated to discuss its .... which indicates that the density of the antibonding state of Ag2−...
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J. Phys. Chem. C 2007, 111, 4042-4046

A DFT Study on Electronic Structures and Catalysis of Ag12O6/Ag(111) for Ethylene Epoxidation W. Gao, M. Zhao, and Q. Jiang* Key Laboratory of Automobile Materials (Jilin UniVersity), Ministry of Education, and Department of Materials Science and Engineering, Jilin UniVersity, Changchun 130022, China ReceiVed: December 9, 2006; In Final Form: January 11, 2007

The electronic structure characteristics of P(4 × 4) or Ag12O6/Ag(111) overlayer structure and its catalysis for ethylene epoxidation are determined using Density Functional Theory (DFT). The adsorption characteristic of ethylene is obtained according to the partial density of state (PDOS) of Ag12O6/Ag(111) overlayer. Moreover, the relative energetic diagrams for the ethylene oxidation process at different adsorption sites are determined using linear synchronous transit/quadratic synchronous transit (LST/QST) tools in DMOL3 code, which show reaction mechanisms and associated barriers for the complete catalytic cycle of conversion of ethylene to ethylene epoxide.

1. Introduction As a noble metal, Ag is one of effective catalysts for ethylene epoxidation.1-2 The subsurface or “bulk-dissolve” O species formed on Ag(111) was suggested to play an essential role in the ethylene epoxidation reaction under ambient and oxygenrich conditions.3-4 Meanwhile, O adsorbed on Ag surface was commonly accepted as active species.5-6 However, the formed surface oxide structures have been argued as a (111) layer of bulk Ag2O,7 or an Ag2O-like oxide overlayer with a stoichiometry of Ag11O6 phase,8 or an Ag9O6 overlayer.9 The latest experimental results and first-principle calculations demonstrate that the structures are comprised of Ag6 motifs, such as P(4 × 4)10-11 and C(3 × 5 x3)10 phases, while the latter is exceedingly similar to the former.10 As a more simple structure, P(4 × 4) structure consists of two Ag6 triangles and six O atoms located in the “trough” between Ag6 triangles. The Ag overlayer with 12 atoms are located approximately above the threefold sites of the underlying Ag(111) substrate, where a half of them is near hcp sites and the others is near fcc sites.10-11 The detailed structure geometry and related sites of O atoms are shown in Figure 1 and Table 1. Three Oup and three Odown atoms are located in each “trough” where the subscripts up and down show the relative large or small distance of O atoms from the substrate surface. This distance is denoted as d′. d′down ) 2.21 Å and d′up ) 3.04 Å, respectively.11 Although the atomic structure of this overlayer is well understood, its electronic structure remains unknown. The epoxidation12 and the oxygen adsorption13 on Ag surfaces, especially for ethylene adsorption and epoxidation on the most stable site of Ag11O6 overlayer, has been studied.14-15 To understand the sequence of elementary steps to form C2H4O, isolating and identifying intermediates are thermodynamically and kinetically accessible using different surface science techniques, such as “frozen in time”,16 temperature programmed desorption and high-resolution election energy loss spectroscopy,17 and Density Functional Theory (DFT) calculations with a total energy minimization technique.18-19 The DFT calcula* Author to whom any correspondence should be addressed. Fax: +86 431 85095876. E-mail: [email protected].

Figure 1. Model structure of Ag12O6/Ag(111) in computer simulation. Figure 1a is a side elevation, and Figure 1b shows an oblique drawing. The gray and the red balls denote Ag and O atoms.

TABLE 1: d and ∆d Values of Ag12O6 Structure on Ag(111) Surface by DFT Calculations and Er Values Calculated by eq 4 Where d Denotes the Height of Oup and Odown from the Uppermost Layer of the Substrate and ∆d ) dup-ddowna Oup Odown a

d (Å)

d′ (Å)

∆d (Å)

∆d′ (Å)

Er (eV)

3.08 2.20

3.04 2.21

0.88

0.85

1.40 1.27

d′11 and ∆d′10 are literature data.

tions are realized by changing the distance of reactants and optimizing the corresponding energy, which is verified by the presence of a single imaginary mode from an additional vibrational frequency analysis.20 After that, a reaction coordinate being accord with experimental observations can be constructed.17 In this work, P(4 × 4) model is selected as a prototype of our work for the partial oxidation of ethylene while C(3 × 5 x3) phase is exceedingly similar to P(4 × 4) and is ne-

10.1021/jp0684729 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/20/2007

Catalysis of Ag12O6/Ag(111)

J. Phys. Chem. C, Vol. 111, No. 10, 2007 4043

glected.10 The catalysis of this surface layer phase is determined by considering the adsorption of ethylene on P(4 × 4) structure. Partial density of states (PDOS) of Ag12O6 phase is calculated to discuss its electronic structure and adsorption characteristics of C2H4. The possible intermediate structures are obtained by DFT calculation. Moreover, to find the possible energy pathway during the process of the ethylene epoxidation on Ag12O6/Ag(111) structure, linear synchronous transit/quadratic synchronous transit (LST/QST) tools in DMOL3 code is used,21-23 which shows also the transition-state in a simple manner and get the barrier energy immediately. Hence, reaction mechanisms and associated barriers for the complete catalytic cycle of C2H2 + O T C2H2O conversion is determined. 2. Theory Methods First principles simulations have been performed using DMOL3 code. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional is employed as the exchange-correlation functional. DFT semicore pseudopots (DSPP) were included via a local pseudopotential for allelectrons calculations, where the effect of core electrons is substituted by a simple potential including some degree of relativistic effects. This technique is computationally inexpensive and is a very useful approximation for elements with atomic number being more than 21.24 To investigate the minimum energy pathway for the ethylene epoxidation, LST/QST tools in DMOL3 code, which have been well validated to find a transition-state structure, are used where GGA with Becke exchange plus Perdew (BP) functional are employed.24 4 × 4 × 1 Monkhorst-pack K-point mesh has been utilized for all geometry optimization calculations. Unless otherwise stated, the supercell contains a slab of three Ag(111) layers.9 A vacuum region being larger than 12 Å is set to separate adjacent slabs. Several test calculations with four layer Ag(111) slabs indicate that the error range is only 2.3% between three and four layer Ag(111) slabs. During the structure optimizations, Ag atoms in the top layer and the overlayer atoms of oxides are allowed to fully relax while two bottom Ag layers are fixed. The calculated structural parameters and related reference data are shown in Table 1 where the simulated parameters of Ag12O6 structure correspond to literature data10-11 with an error range of 3.4%, which confirms the validity of our calculation. The adsorption energy E is computed by

E ) - (Ex/s - Ex - Es)

(1)

where the subscript x denotes O atoms or C2H4 molecules for different adsorbed structures, and s shows Ag12O6/Ag(111) structure. The interaction energy Eint is calculated for each adsorption case, which represents the bonding strength between C2H4 and the substrate in its deformed geometry14

Eint ) - (Ex/s - E′x - E′s)

(2)

where E′x and E′s are, respectively, the energies of ethylene and substrate already in its deformed geometry. Since Eint ) E + Edef14 where Edef denotes the deformation energy of the substrate after adsorption due to the relaxation of Ag12O6/Ag(111) structure, there is

Edef ) Eint - E

(3)

To investigate energetic difference of O atoms in each trough, the idea of “removed energy” Er is employed, which denotes

Figure 2. PDOS curve of Ag12O6 structure on Ag(111). Subscripts 1 and 2 denote the Ag atoms bonded with Oup and with both Oup and Odown.

the energy required to remove an O atom from the surface to vacuum25

Er ) - (E′O/Ag(111) - EO/Ag(111) - EO)

(4)

where E′O/Ag (111) and EO/Ag (111) denote the total energies of the considered supercell with and without the removed O atom. EO is the total energy of a free O atom where the spinpolarization energy is taken into account. Er reflects the bond strength of the removed O atom to the surface,26 or the adsorption energy of an O atom onto a supercell that has contained other O atoms.13 3. Results and Discussion PDOS of Ag12O6 surface layer phase is shown in Figure 2 where O and Ag hybridize mainly via Ag-4d and O-2p orbitals with additional hybridization of Ag-5s, which is similar to other O/Ag(111) structures.26 Let subscripts 1 and 2 denote two bonding states of Ag where the former denotes the Ag atoms bonding only with Oup while the latter shows the Ag atoms bonding with both Oup and Odown. There are two main regions with high electron density, which are at -1.26 eV and -0.37 eV below the Fermi level, corresponding to the bonding and the antibonding states.14,26 Strong covalent bonding, which requires unoccupied antibonding states, is prevented since the antibonding states are largely occupied.26 This is propitious to remove O atom from the overlayer. As shown in Figure 2a, there is a small peak at -1.74 eV of Ag2-4d compared to Ag14d, which indicates that the density of the antibonding state of Ag2-O is larger than that of Ag1-O. There is a downshift of 2p band center from Oup to Odown in Figure 2b due to the weakening of metallic characteristics of antibonding states.14 PDOS in Figure 2b is rather similar to that of O/Ag(111) structure with a similar local bonding where there is only one peak outside Ag-4d band.26 This figure differs from the cases of oxide-like structures13 and Ag2O (111) although early works considered that Ag surface oxide is similar to a (111) layer of bulk Ag2O.7 In light of Table 1, Er,up and Er,down values are 1.40 and 1.27 eV, respectively. Since Oup is fourfold bonded with around Ag6 triangles while Odown is threefold, Er,up > Er,down is understandable due to their different bonding characteristics. Note that if

4044 J. Phys. Chem. C, Vol. 111, No. 10, 2007

Gao et al.

Figure 3. Optimized intermediate stable structures after C2H4 is interacted at adsorption sites of Oup-Ag1(Figure 3a), Oup-Ag2 (Figure 3b), OdownAg1(Figure 3c) and Odown-Ag2(Figure 3d). C2H4 is adsorbed at those sites with its CdC bond paralleling O-Ag bond.14 The black and white balls show C and H atoms. The five-membered ring structures in yellow are shown in Figures 3b and 3c, which contain an O, two C, and two Ag atoms.

TABLE 2: E, Eint and Edef Values in Terms of Equations (1-3) for Four Ethylene Adsorption Sites in eV E Eint Edef

Oup-Ag1

Oup-Ag2

Odown-Ag1

Odown-Ag2

0.10 0.16 0.06

0.16 0.62 0.46

0.05 0.25 0.20

0.004 0.004 0

Odown transforms from threefold to fourfold, the total energy of the system increases 0.003 eV. EC2H4 values for C2H4 adsorbed on Ag1 and Ag2 are 0.119 and 0.121 eV, respectively, in terms of eq 1, which are consistent with present E values of 0.092-0.103 eV between C2H4 and Ag surface,27-29 and much smaller than Er,up and Er,down. Thus, the weakly adsorbed C2H4 on P(4 × 4) surface layer under reaction conditions is in a motion state and is continuously adsorbed and desorbed.17 The adsorption process can be described as follows: One of two C atoms in C2H4 is first adsorbed at an Ag site, which satisfies the tetra-valence behavior of C.17 After that another C atom approaches an O site for further reaction.14-15 Using the total energy minimization technique, the ethylene epoxidation process on P(4 × 4) surface layer with the corresponding optimized intermediate structure is described and shown in Figure 3. In the figure, both ends of an O-C-C backbone are set to be attached on the surface, in which the C atom bonded directly to O atom is denoted as C1 while another denoted as C2 is bonded to Ag atom. dO-C1 ) 1.60 Å is first set as an initial value for structure optimization at four different adsorption sites, the results are shown in Figures 3. Three of them, namely, Oup-Ag1, Oup-Ag2, and Odown-Ag1, could bond the substrate at the adsorption sites (Figures 3a-3c). However, even dO-C1 is dropped to 1.20 Å, as shown in Figure 3d, the bonding at Odown-Ag2 site is still absent, which implies the absence of bonding there. The computed E, Eint, and Edef values for all related sites are shown in Table 2. When stabilities of different sites for C2H4 adsorption are compared, Oup-Ag2 > Oup-Ag1 > Odown-Ag1 > Odown-Ag2. This order is induced by electronic differences of Oup, Odown, Ag1, and Ag2. In C2H4 adsorption, the main

stabilizing interaction occurs between the vacant 2π* (located around +3 eV) orbital of ethylene and Ag-4d orbital,14 as seen in Figure 2. The stronger antibonding state of Ag2-O favors the bonding of Ag2-O to vacant 2π* and results in stronger molecule-surface interaction.14 Thus, compared with Ag1, Ag2 is the preferential adsorption site. In another side, Odown is less accessible than Oup due to the geometric argument,14 Oup is therefore the preferential adsorption site. The both bring out the most stable adsorption site of Oup-Ag2 for ethylene. Since Eint values are small, the combinations between C2H4 and O in the overlayer at Oup-Ag1 and Odown-Ag2 sites are very weak while Edef is nearly negligible.14 However, those at Oup-Ag2 and Odown-Ag1 sites shown in Figures 3b and 3c should be chemically bonded by C1-O bond formation.14 dup changes from 3.08 Å to 4.07 Å (Figure 3b) and ddown from 2.20 Å to 3.40 Å (Figure 3c), respectively, due to weakening of O-Ag bonds and moving of O further away from the substrate.17 Since Ag-O bond becomes weaker, O-C1 bonds and weaker adsorptions of O-C1-C2 structures on the substrate are formed, which agree with the bond order conservation arguments.30 The structure as an intermediate with the adsorption of a five-membered ring containing a backbone O-C-C linkage and two Ag atoms (OMME) is shown in Figures 3b,c, which has been found experimentally.31 Determined bond length values of OMME l are also consistent with literature data, which are denoted as l3 to l6 2,17-18,31 and shown in Table 3. Note that dup in Figure 3a and ddown in Figure 3d are approximately immobile, which imply little interaction occurring there where no intermediate is formed. Since the epoxidation process could only take place at OdownAg1 and Oup-Ag2 sites, the energetic profiles for C2H4O formation at the both sites are further studied and shown in Figure 4 where the epoxidation occurs with a two-step process via OMME. At Odown-Ag1 site shown in Figure 4.1, C2H4 plus Ag12O6 structure is taken as the initial state. C2H4 is adsorbed weakly at Ag1 site as discussed above (EC2H4 ) 0.119 eV). Let Eb denote the barrier energy from one state to another state, Eb to produce

Catalysis of Ag12O6/Ag(111)

J. Phys. Chem. C, Vol. 111, No. 10, 2007 4045

TABLE 3: Bond Lengths of Intermediates on Ag12O6/ Ag(111) Structurea C1-O C1-C2 C2-Ag O-Ag Ag-Ag

l1 (Å)

l2 (Å)

l3 (Å)

l4 (Å)

l5 (Å)

l6 (Å)

1.410 1.527 2.244 2.245 3.150

1.461 1.509 2.286 2.356 3.117

1.41 1.516 2.255 2.155 3.002

1.51 1.43

1.44 1.51 2.26 2.16 3.00

1.49 1.52 2.27 2.24

2.28

a l1 and l2 denote those of C2H4 being adsorbed on Oup-Ag2 and Odown-Ag1 sites. Literature values of l calculated are also shown, which are when C2H4 is adsorbed on O/Ag(111) or O/Agcluster structure, which are denoted as l3,2 l4,17 l5,18 and l631.

Figure 4. Relative energy diagram for the conversion of C2H4 to C2H4O at Odown-Ag1 (Figure 4.1) and Oup-Ag2 sites (Figure 4.2) on Ag12O6/ Ag(111) overlayer using LST/QST tools. The panels from a to g indicate the corresponding states, which is in turns the gas-phase C2H4 and Ag12O6 structure, C2H4/Ag12O6 structure, the first transition state TS1 and Ag12O6 structure, OMME/Ag12O5 structure, the second transition state TS2 and Ag12O5 structure, C2H4O/Ag12O5 structure, and gas-phase C2H4O and Ag12O5 structure. The structures shown correspond to transition states and OMME where the arrows depict motion of atoms during the reaction. The numbers shown denote the energetic change values Eb in eV between adjacent two states.

OMME from the C2H4 adsorbed state is 0.23 eV, which is between Eb ) 0.32 eV on O/Ag(111)15 and Eb ) 0.17 eV on O/Agcluster structure.17 At the first transition state TS1, the

energetic level is 0.11 eV larger than that of the initial state where a C-O bond is created through a lateral shift of C2H4 toward Odown. Meanwhile, Odown moves upward. Let subscript denote the corresponding bond, lC-C of C2H4 at TS1 shown in Figure 4.1c and Table 4 is elongated from 1.330 to 1.377 Å, which is consistent with that of 1.38 Å on O/Agcluster structure.17 After TS1 state is reached, OMME is built with energetic drop of 0.05 eV from the initial state. In the next step, C2H4O is formed from OMME via the second transition state TS2 with Eb ) 0.15 eV, which is smaller than experimental and simulated values of Eb ) 0.20 eV,20 Eb )0.92 eV adsorbed on O/ Ag(111)15, and Eb ) 0.18 eV adsorbed on O/Agcluster structure.17 In Figure 4.1e, C2 moves away from Ag1 toward Odown atom, C2-Odown bond begins to form with lC2-Odown ) 2.155 Å. Compared with OMME, the lC2-Ag1 bond at TS2 state is elongated from 2.286 to 3.439 Å while the lC-C bond is shortened from 1.509 to 1.460 Å. Thus, the product of this elementary step is C2H4O.17 The energetic level at TS2 is 0.10 eV higher than that of the initial state. Thus, combination of C2H4O and the oxygen-deficient oxide Ag12O5 is weak and has only a size of 0.06 eV. The C2H4O + Ag12O5 as the product of C2H4+Ag12O6 is 0.02 eV more stable than C2H4 + Ag12O6. Considering experimental Eb values with broad range of 0.21.1 eV,15,20 this process could be realized. Note that the extraction of an O from Ag12O6 makes little reconstruction of the surface oxide. To complete this catalytic cycle, Eb for O2 dissociation on Ag12O5 examined is 0.63 eV, which agrees with Eb ) 0.64 eV on O/Ag(111).15 At the Oup-Ag2 site, the reaction process of C2H4 is analogical with that at Odown-Ag1 site as shown in Figure 4.2. C2H4 is adsorbed weakly on Ag2 site (0.121 eV), as like as that at Ag1 site. The energetic state of TS1 with Eb ) 0.29 eV is higher than that at Ag1 site and is 0.17 eV higher than that of the initial state. At TS1, lC-C ) 1.413 Å and lC-O ) 1.980 Å. OMME obtained from this step is more stable than that at Odown-Ag1 site and is 0.16 eV more stable than the initial state. In the next step, the energetic level is 0.20 eV larger than that of the initial state and Eb ) 0.36 eV, which is twice that at Odown-Ag1 site. The structure of TS2 shown in Table 4 is consistent with those at Odown-Ag1 site and on O/Agcluster structure. The obtained C2H4O plus Ag12O5 with E ) 0.06 eV are 0.12 eV less stable than that at the initial state. Hence, the whole reaction of C2H4O at Oup-Ag2 site is uplifted in energy. For the reaction of Ag12O5 + OdAg12O6, Eb ) 0.41 eV at Oup site, which is smaller than that at Odown site. Namely, Oupdeficient is easier to recover than Odown-deficient. Note that the structure at the Oup-Ag1 site is also calculated by LST/ QST tools where C2H4O cannot be built due to absence of an intermediate for its low Eint value as discussed above. Because a structure at the Odown-Ag2 site has not even formed, the corresponding calculation for Eb is neglected. In summary, Our calculations confirm that the ethylene epoxidation occurs via OMME.15,17,20,31 lC-C is elongated from C2H4 to OMME and shortened from OMME to C2H4O where OMME dissociates easily.

TABLE 4: Bond Lengths of Transition States on Ag12O6/Ag(111) Structure at Odown-Ag1 and Oup-Ag2 Sitesa Odown-Ag1 C1-O C1-C2 C2-O C2-Ag a

Oup-Ag2

O/Agcluster

l at TS1(Å)

l at TS2 (Å)

l at TS1 (Å)

l at TS2 (Å)

l at TS1 (Å)

l at TS2 (Å)

2.147 1.377 2.958 2.507

1.474 1.460 2.155 3.439

1.980 1.413 2.779 2.511

1.507 1.465 2.203 3.786

1.94 1.38

1.56 1.45 1.99

Available simulation results on O/Agcluster structure are also shown.17

4046 J. Phys. Chem. C, Vol. 111, No. 10, 2007 At two sites in Figure 4, the ethylene epoxidation mechanisms are reasonably semblable in terms of both structures and energies. The initial state for both possible processes is identical. The two transition states with larger Eb values at the Oup-Ag2 site, however, are much harder to reach while OMME at OupAg2 site is more stable than that at Odown-Ag1 site. It is known that a good catalyst is characterized by low activation energy and weak bonding of the intermediates.32 For the intermediate in Figure 4, at Odown-Ag1 site, Eb ) 0.16 eV to reform the adsorbed C2H4, which is larger than that to form C2H4O (Eb )0.15 eV). The corresponding values at the Oup-Ag2 site are 0.33 and 0.36 eV, respectively. This energetic difference brings out the preferred Odown-Ag1 site for the ethylene epoxidation in comparison with the Oup-Ag2 site. Moreover, the whole reaction of C2H4O at the Odown-Ag1 site is degressive in energy, being opposite to that at the Oup-Ag2 site. Also, the energetic difference at the two sites after the process is 0.14 eV, which is consistent with the difference of Er,up - Er,down ) 0.13 eV as shown in the above. However, Oup is easier to recover for its lower O2 dissociation barrier, which leads to larger recover rate and is beneficial for the process. Thus, in reality, both sites should contribute the ethylene epoxidation process while the other two sites are impossible. Our calculations are performed at T ) 0 K. The ab initio MC simulations11 demonstrate that the asymmetry for the oxygen positions is lifted at T ) 700 K and that the average position is lowered, which is also inline with the experimental findings.11 Thus, the reaction path should be a little different from our results at higher temperatures. However, our calculation only concerns with the reaction path at 490 K with the corresponding Ag12O6/Ag(111) structure, which is identical with that prepared by O2 at 490 K or NO2 at 480 K in the literature,11 and is stable under actual ethylene epoxidation temperatures (T ) 473-573 K).5 Thus, our calculated reaction path is valid for the actual ethylene epoxidation conditions. Indeed, if further studies using ab initio thermodynamic calculations or kinetic Monte Carlo calculations are carried out, where realistic gas pressures, finite temperatures and adsorption-desorption kinetics are taken into account, the above results could be confirmed or improved, which should be our future works. 4. Conclusions In conclusion, the electronic structure of Ag12O6/Ag(111) overlayer and the corresponding ethylene epoxidation process have been studied by DFT. The electronic structure of Ag12O6/ Ag(111) overlayer differs from that of oxide-like structures and Ag2O (111), but rather similar to that of O/Ag(111) one with a similar local bonding. Oup-Ag2 and Odown-Ag1 sites dominate

Gao et al. the ethylene epoxidation process while Oup-Ag1 and OdownAg2 sites have little effects on it. Acknowledgment. The financial supports from National Key Basic Research and Development Program (Grant No. 2004CB619301) and “985 Project” of Jilin University are acknowledged. References and Notes (1) Saravanan, C.; Salazar, M. R.; Kress, J. D.; Redondo, A. J. Phys. Chem. B 2000, 104, 8685. (2) Mavrikakis, M.; Doren, D. J.; Barteau, M. A. J. Phys. Chem. B 1998, 102, 394. (3) Nagy, A.; Mestl, G.; Herein, D.; Weinberg, G.; Kitzelmann, E.; Schlo¨gl, R. J. Catal. 1999, 182, 417. (4) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (5) Stegelmann, C.; Schiødt, N. C.; Campbell, C. T.; Stoltze, P. J. Catal. 2004, 221, 630. (6) Hendriksen, B. L. M.; Frenken, J. W. M. Phys. ReV. Lett. 2002, 89, 046101. (7) Rovida, G.; Pratesi, F.; Maglietta, M.; Ferroni, E. Surf. Sci. 1974, 43, 230. (8) Carlisle, C. I.; King, D. A. Phys. ReV. Lett. 2000, 84, 3899. (9) Michaelides, A.; Reuter, K.; Scheffler, M. J. Vac. Sci. Technol. A 2005, 23, 1487. (10) Schnalt, J.; Michaelides, A.; Knudsen, J.; Vang, R. T.; Reuter, K.; Lægsgaard, E.; Scheffler, M.; Besenbacher, F. Phys. ReV. Lett. 2006, 96, 146101. (11) Schmid, M.; Reicho, A.; Stierle, A.; Costina, I.; Klikovits, J.; Kostelnik, P.; Dubay, O.; Kresse, G.; Gustafson, J.; lundgren, E.; Andersen, J. N.; Dosch, H.; Varga, P. Phys. ReV. Lett. 2006, 96, 146102. (12) Avdeev, V. I.; Zhidomirov, G. M. Surf. Sci. 2001, 492, 137. (13) Li, W. X.; Stampfl, C.; Scheffler, M. Phys. ReV. B 2003, 67, 045408. (14) Bocquet, M. L.; Sautet, P.; Cerda, J.; Carlisle, C. I.; Webb, M. J.; King, D. A. J. Am. Chem. Soc. 2003, 125, 3119. (15) Bocquet, M. L.; Michaelides, A.; Loffreda, D.; Sautet, P.; Alavi, A.; King, D. A. J. Am. Chem. Soc. 2003, 125, 5620. (16) Norskov, J. K. Nature 2001, 414, 405. (17) Linic, S.; Barteau, M. A. J. Catal. 2003, 214, 200. (18) Jones, G. S.; Mavrikakis, M.; Barteau, M. A.; Vohs, J. M. J. Am. Chem. Soc. 1998, 120, 3196. (19) Medlin, J. W.; Marteau, M. A. J. Phys. Chem. B 2001, 105, 10054. (20) Linic, S.; Barteau, M. A. J. Am. Chem. Soc. 2002, 124, 310. (21) Delley, B. J. Chem. Phys. 1990, 92, 508. (22) Delley, B. J. Phys. Chem. 1996, 100, 6017. (23) Delley, B. J. Chem. Phys. 2000, 113, 7756. (24) The help of the Materials studio modeling. (25) Li, W. X.; Stampfl, C.; Scheffler, M. Phys. ReV. B 2003, 68, 165412. (26) Li, W. X.; Stampfl, C.; Scheffler, M. Phys. ReV. B 2002, 65, 075407. (27) Backx, C.; deGroot, C. P. M.; Biloen, P. Appl. Surf. Sci. 1980, 6, 256. (28) Zhou, X. L. J. Phys. Chem. 1992, 96, 7703. (29) Stacchiola, D.; Wu, G.; Kaltchev, M.; Tysoe, W. T. Surf. Sci. 2001, 486, 9. (30) Shustorovich, E. M. Surf. Sci. Rep. 1986, 6, 1. (31) Linic, S.; Medlin, J. W.; Barteau, M. A. Langmuir 2002, 18, 5197. (32) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Bahn, S.; Hansen, L. B.; Bollinger, M.; Bengaard, H.; Hammer, B.; Sljivancanin, Z.; Mavrikakis, M.; Xu, Y.; Dahl, S.; Jacobsen, C. J. H. J. Catal. 2002, 209, 275.